Started the gym and lost my period?

im 18 years old and my cycle is like clockwork, 28 days on the dot for years now. I never did that much exercise during school, but now that iv finished, iv joined the gym. i do 20 - 30 mins jogging, then upper and lower body exercises on alternating days. from the time i started exercising to now, i was supposed to have 2 periods, but they just haven't come. i read an article on exercise induced amenorrhea, and it said that women who dont eat enough lose their periods because their bodies cant fuel exercise and regular menstrual cycles. Ever since i started working out my appetite had greatly increased so iv been eating heaps. Does anybody know why this could be happening? (btw im definitely not pregnant)